With the increasing demand for electrical energy there has been a correspondingly increased interest in methods of producing such energy by geothermal means through the utilization of the natural heat of the earth's interior. Numerous geothermal energy recovery processes have been put forward. These processes all face the problem of obtaining a sufficient amount of heat at a temperature high enough (i.e. a minimum of about 350.degree. F.) for process steam or electrical power production. This necessitates the search for anomolous geological formations which have both relatively high temperatures close to the surface and a sufficient reservoir of fluid to make development worth while.
Geyser basins, which provide "dry" steam, are very rare. Furthermore, such "dry" stream methods are not easily adaptable to being operated intermittantly. Certain natural geothermal fluids, such as hot water, are much more abundant but pose corrosion or plugging problems, because of solids dissolved therein, to equipment. Other geothermal methods propose utilizing geopressured water which is located, with much difficulty, at greater depths. Because of the net withdrawal of fluids, all three of these methods are, like oil and gas, depleting resources. In addition, these methods pose environmental problems associated with the release of noxious gases into the atmosphere, the disposal of large amounts of precipitated solids, and the possibility of earth subsidence or earthquakes due to the withdrawal of subterranean fluids.
The Los Alamos Scientific Laboratory's (Energy Research and Development Administration) hot rock process requires drilling relatively deep (i.e., about 10,000 to 15,000 feet) wells into hot, hard rock (such as granite rather than sedimentary rock) and the continued cracking of that rock by cold working fluid to provide enough fresh hot surface for economical heat transfer. This process has the advantage of providing its own fluid in a closed system. However, if further cracking of the hot rock cannot take place, heat transfer will be limited by the slow rate of conduction and, therefore, the life of such a well, and the cost of the energy obtained therefrom, will be difficult to estimate.
In recent years there has been interest in the uses that could be made from the geothermal heat that is found in certain geological formations such as spires or diapirs or "domes" of crystalline rock salt. These domes, which are found in numerous places throughout the world, including the Gulf Coast of the United States and Mexico are, in many instances, found at a reasonable depth within reach of conventional drilling equipment.
One such method of using the heat energy of rock salt domes is taught in U.S. Pat. No. 3,676,078. This method teaches dissolving a cavity within the depths of a salt dome, transferring heat to a heat exchange fluid in the cavity, removing the heat exchange fluid from the cavity, recovering the heat energy contained therein and recirculating the heat exchange fluid back into the cavity. This method has several drawbacks. First, because of the relatively high temperatures that exist at the depths to which U.S. Pat. No. 3,676,078 teaches to excavate, the rock salt will behave plastically. At these depths, unless a counter-pressure is maintained, the normal pressure due to the weight of salt and its overburden is sufficient to cause the plastic salt to flow and to thereby close the cavity. Furthermore, this method limits the choice of heat exchange fluid, in that such fluid will come in direct contact with the salt and it is therefore desirable that the rock salt be insoluble in the fluid. In addition, the above method requires mining, for each salt dome cavity in operation, relatively large amounts of salt. From an economical and operational standpoint, it is questioned whether such an extensive excavation is prohibitively large when compared to the amount of heat energy recovered by this method.
One advantage of the present invention is that it provides for a practical geothermal energy recovery process that is characterized by the knowledge of where to drill, since the geology of salt domes has been extensively researched, and the ability to calculate how long a given heat output can be maintained from a given salt formation.
Another advantage of the present invention is that it provides for a geothermal energy recovery process in which the primary fluid used in the process is self-contained and is not exposed to the underground environment, thereby effectively eliminating the danger of having dissolved solids corrode the mechanisms and conduits employed or precipitate out therein.
A further advantage of this invention is that, because it is dependent upon conduction heat transfer, it can be operated intermitantly. Such intermittant operation will allow the geothermal energy derived therefrom to be used to provide peaking power and thus make economically justifiable the installation of oversized equipment (i.e., a turbine, generator and condensor) while new wells are still being added to the geothermal power plant.